In some of conventional automobile battery power circuits, a battery group and a capacitor group are connected in series, for example, as disclosed in JP 2002-218667 A (FIGS. 3 and 10).
In the aforementioned JP 2002-218667 A, the battery group is composed of a single battery with a rated voltage of 12 V, and the capacitor group is composed of a plurality of capacitors. A DC/DC converter is connected between both ends of the capacitor group and the battery group, and supplies the battery group with a power by lowering a voltage of the capacitor group. Here, the battery group is intended to supply a power to an electrical component with a rated voltage of 12 V, and loads are connected to both ends of the battery group.
In a conventional unit, a power conversion circuit starts operating at the time of a start (in starting an idling stop operation), and a bidirectional DC/DC converter is operated at the same time to transmit a power from the battery group to the capacitor group, whereby the amount of a current flowing through the capacitor group can be reduced. Therefore, the capacitance of the capacitor group can be reduced. After the power conversion circuit has been turned off, the bidirectional DC/DC converter recharges the capacitor group until its voltage reaches a predetermined voltage, thus preparing for a subsequent restart.
As shown in FIG. 10 of JP 2002-218667 A, a circuitry of the bidirectional DC/DC converter of the conventional power unit is provided with two MOSFETs that perform switching at a high frequency (50 to 200 kHz) in boosting and lowering a voltage. In the first MOSFET that performs switching in boosting a voltage, a drain terminal is connected to a high-voltage-side terminal of a capacitor, a source terminal is connected to one terminal of a choke coil, and a gate terminal is connected to a control circuit. In the second MOSFET that performs switching in lowering a voltage, a drain terminal is connected to the choke coil and the source terminal of the first MOSFET, a source terminal is connected to a low-voltage-side terminal of the battery group, and a gate terminal is connected to the control circuit. The other terminal of the choke coil is connected to a high-voltage-side terminal of a battery group and one electrode of a smoothing capacitor, and the other terminal of this capacitor is connected to a low-voltage-side terminal of the battery group.
The power unit supplies energy from the battery group and the capacitor group to the power conversion circuit in starting an idling stop operation, and the power conversion circuit operates. A motor thereby rotates, and an automobile starts moving. The motor causes a rise to an idling rotational speed, and an engine is ignited, so that a shift in driving source is made from the motor to the engine. At the time of a start, the motor operates for 0.2 to 1 second approximately. On the other hand, the motor also functions as a generator. Therefore, while the automobile is running, the power unit is in operation. This operation time is much longer than an operation time at the time of the start.
Because of a short operation time, the amount of heat generated at the time of the start is sufficiently smaller than the amount of heat generated during a power generation operation. The amount of instantaneous heat generation at the time of the start is very large, but can be sufficiently absorbed with a heat capacity of the unit. Accordingly, heat generation at the time of power generation is taken into account in thermally designing the power unit (designing the unit such that its temperature does not exceed an allowable temperature). In the conventional unit, the DC/DC converter is driven at a high frequency at the time of power generation so as to lower energy generated on the high voltage side (Vf3 in the drawing) to the level of a 12V battery. When high-frequency switching is performed, a switching loss increases and thus causes heat generation in a DC/DC converter portion. Because a power conversion circuit portion operates in a diode rectifying mode at the time of power generation, its frequency is low even when a switching operation is performed. The amount of heat generated by the power conversion circuit portion at this moment is small. Therefore, heat generation in the DC/DC converter portion must be sufficiently considered in thermally designing the unit. To keep the temperature of the unit at an allowable value, such measures as enlarging a radiation fin of the unit and mounting a forced air cooling fan or disposing a water cooling radiation fin on a lower face of the unit and turning it into a water cooling mechanism are required. Those measures bring about a rise in cost as well as an increase in size of the unit. Thus, the conventional power unit has a problem of being bulky and costly.